1. Field of the Invention
The present invention relates to a DC power source apparatus, and particularly, to a power converting transformer used for a DC power source apparatus.
2. Description of the Related Art
FIG. 1 is a circuit diagram illustrating a DC power source apparatus according to a related art. The apparatus includes a switching element Q1 made of, for example, a MOSFET that is turned on/off to intermittently convert a DC input voltage from a DC power source E into high-frequency power, a transformer 1a having primary windings P1 and P2 between the switching element Q1 and a positive electrode of the DC power source E, to transmit the high-frequency power from the primary side to the secondary side, a rectifying-smoothing circuit consisting of a diode D1 and a smoothing capacitor C1, to convert the high-frequency power transmitted to the secondary side into a DC output voltage and supply the DC output voltage to a load, an output voltage detector 3 to detect the DC output voltage, compare the detected voltage with a reference voltage, and output an error signal, and a controller 5 to control an ON/OFF period of the switching element Q1 according to the error signal. The apparatus controls ON/OFF of the switching element Q1 so as to supply a predetermined output voltage to the load.
A voltage induced by a tertiary winding D is rectified and smoothed with a diode D2 and a capacitor C2 and is supplied as a source voltage to the controller 5.
FIG. 2 is a sectional view illustrating a structure of the transformer 1a of the DC power source apparatus of FIG. 1, FIG. 3 is a schematic view illustrating the windings of the transformer 1a, and FIG. 4 is a view illustrating parasitic capacitances among the windings of the transformer 1a. 
In FIGS. 1 and 2, the transformer 1a has a core 11a made of magnetic material and a bobbin 13 fitted to the core 11a. The bobbin 13 is wound with, sequentially from an inner side, the first primary winding P1, a secondary winding S, the second primary winding P2, and the tertiary winding D (not illustrated). The core 11a defines two magnetic paths with a center leg being common to them. The first primary winding P1 has two winding layers P1-1 and P1-2 and the second primary winding P2 has two winding layers P2-1 and P2-2.
A sequence of winding the windings on the bobbin 13 will be explained. Downwardly from a left end of the bobbin 13, an element wire starts to be wound to form the winding layer P1-1. At a right end of the bobbin 13, the wire is turned back to form the winding layer P1-2 over the winding layer P1-1, thereby forming the first primary winding P1. On the winding layer P1-2, the secondary winding S is wound. Thereafter, the winding layers P2-1 and P2-2 are wound in the same direction as the winding layers P1-1 and P1-2, to form the second primary winding P2.
To improve productivity, the windings of the transformer 1a are generally wound in the same direction. As illustrated in FIG. 1, the first and second primary windings P1 and P2 are connected in parallel with each other. As illustrated in FIGS. 2 and 3, the secondary winding S is interposed between the first and second primary windings P1 and P2, to improve the degree of magnetic coupling between the primary windings P1 and P2 and the secondary winding S. As illustrated in FIG. 4, a parasitic capacitance C112 is present between the winding layers P1-1 and P1-2, a parasitic capacitance C12S between the winding layer P1-2 and the secondary winding S, a parasitic capacitance C21S between the secondary winding S and the winding layer P2-1, and a parasitic capacitance C212 between the winding layers P2-1 and P2-2.
In FIGS. 1 and 3, the winding layer P1-1 of the first primary winding P1 and the winding layer P2-1 of the second primary winding P2 adjacent to the secondary winding S are connected to the switching element Q1.
The switching element Q1 is always turned on and off, and at each time of the on/off operation, greatly changes potential. This potential change is applied to the first and second primary windings P1 and P2, to pass high-frequency currents to the secondary side through the parasitic capacitance C12S between the winding layer P1-2 of the first primary winding P1 and the secondary winding S and the parasitic capacitance C21S between the winding layer P2-1 of the second primary winding P2 and the secondary winding S.
The high-frequency currents due to the potential change created by the switching element Q1 circulate through a path extending along the primary windings P1 and P2, the secondary winding S, the secondary-side circuit, the ground, parasitic capacitance between the primary-side circuit and the ground, the primary-side circuit, and the primary windings P1 and P2. The high-frequency currents passed to the ground cause common-mode noise. The common-mode noise leaks to the DC power source E and radiates into the air, to badly affect other devices.
Considering a stable potential line Ls as a reference, the DC input voltage from the DC power source E is applied to a minus side of the primary windings P1 and P2 of the transformer 1a when the switching element Q1 turns on, and when the switching element Q1 turns off, a flyback voltage occurs to a plus side of the primary windings P1 and P2. Namely, a terminal of the primary windings P1 and P2 of the transformer 1a connected to the switching element Q1 is subjected to a largest potential change and the other terminal thereof connected to the stable potential line Ls is subjected to no potential change.
The parasitic capacitances between the primary windings P1 and P2 and the secondary winding S become larger as the windings become closer to one another. Accordingly, high-frequency currents passing through the parasitic capacitances among the primary windings P1 and P2 and the secondary winding S become larger if the terminal of the primary windings P1 and P2 connected to the switching element Q1 is positioned closer to the secondary winding S.
According to the related art illustrated in FIGS. 1 to 4, winding starts (indicated with black dots) of the primary windings P1 and P2 are connected to the switching element Q1. The winding layer P1-1 involving the winding start of the primary winding P1 is away from the secondary winding S. The winding layer P2-1 involving the winding start of the primary winding P2 is adjacent to the secondary winding S. Accordingly, a high-frequency current passing from the winding start of the primary winding P2 through the parasitic capacitance C21S to the secondary winding S is large. In FIG. 1, white arrows indicate high-frequency currents, the thicker the arrow the larger the current.